1. Field of the Invention
This invention relates to a method of regenerating adsorbents for removing toxic pollutants from air. More specifically, it relates to a method of restoring the ability of metal oxide-impregnated activated carbon referred to as whetlerite to remove cyanogen chloride from air.
2. Description of the Prior Art
Whetlerites are activated carbon-based composites impregnated with ammoniacal solutions of divalent copper and monovalent silver, with or without hexavalent chromium, in various concentrations. Optionally, triethylenediamine (TEDA) is added to prolong the service life of the adsorbent and resist the adverse effects of humid environments. Whetlerites adsorb a wide variety of toxic air pollutants, in particular cyanogen chloride, cyanogen, and hydrogen cyanide. They have been used as charcoal filters for gas masks for personnel protection in military combat and in polluted atmospheres, as well as for air intake filters for building air conditioning systems.
It has been found convenient to express the ability of whetlerite and other adsorbents to adsorb toxic air pollutants, in terms of its ability to remove cyanogen chloride, ClCN, from air under standardized conditions. These are as follows:
TABLE 1 ______________________________________ Standard Test Conditions for Evaluating Carbon Adsorbents ______________________________________ Inlet cyanogen chloride concentration 4 mg/L = 1560 ppmv End point breakthrough 0.008 mg/L = 3.2 ppmv Continuous air flow 1630 ml/min Sample bed depth 30 mm Contact time 0.31-0.35 second Moisture in air 80% relative humidity Prehumidification 16 hours ______________________________________
The ability to adsorb cyanogen chloride is expressed in terms of the time to the breakthrough of cyanogen chloride in the effluent of the adsorbent bed.
In the course of time, whetlerite degrades as it adsorbs impurities form the atmosphere, and this degradation is reflected in a decreased ability to adsorb toxic pollutants. This degradation is attributed to the transformation of the copper in the whetlerite, originally in the form of bicarbonate, into the oxide, and the reduction of chromate, originally present as ammonium chromate, into a chromous compound (Cr III).
Several methods of restoring adsorptive capacity to whetlerite and other adsorbents are known. U.S. Pat. No. 4,094,815 relates to the regeneration of activated carbon by passing a mixture of gases and/or vapors capable of reacting exothermically through the bed of activated carbon. The exothermic heat liberated heats the carbon and strips the adsorbed material. The gaseous mixture may be chlorine plus either carbon monoxide or hydrogen. U.S. Pat. No. 4,656,148 relates to a process for the reactivation of a catalyst containing iron oxides for the removal of NOx, oxides of nitrogen, from stack gases by passing carbon monoxide at elevated temperature through the catalyst bed. Two catalyst beds are used. They alternate between NOx removal and regeneration. U.S. Pat. No. 4,801,311 relates to a chromium-free activated carbon of the whetlerite type for removing toxic gases such as hydrogen cyanide, cyanogen, and cyanogen chloride. This chromium-free whetlerite, which contains 1-7.5% of triethylenediamine, is as active as chromium- containing whetlerite.
None of the methods in the prior art restore degraded whetlerites for adsorbing toxic air contaminants such as cyanogen chloride. Therefore, there is a need for such a method, and more particularly an economical method for regenerating whetlerites in situ.